1. Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka 560
2. Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4Ml, Canada
3. Institut Laue-Langevin, 156X 38042 Grenoble Cedex, France
4. Neutron Division, Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, England